Planar transformers are commonly constructed using windings in printed circuit boards, metal stampings, or a combination of the two. Interleaving between primary windings and secondary windings is performed to reduce leakage inductance. The windings on different layers are connected in combinations of series and parallel connections to achieve the correct turns ratio.
Traditionally, the windings connected in parallel are of equal copper weight, equally distributing the current between the windings. Such a design can encounter several drawbacks. For example, in the case of a medium to high power transformer, this type of construction needs circuit boards that contain many layers of heavy copper. As a result, it is difficult and expensive to have a planar transformer of this construction fully imbedded in the host circuit board. The heavy copper weight used in this construction may further increase the minimum trace/space of the other circuitry that is resident on the circuit board. Moreover, a multi-layer circuit board with heavy copper weight may also dramatically increase the fabrication cost of the printed circuit board.
Therefore a need exists for a transformer that is economical and at the same time exhibits desirable conversion properties.